1. Field of the Invention
The present invention relates to a wafer processing method of dividing a wafer along a plurality of crossing streets formed on the front side of the wafer to thereby partition a plurality of regions where a plurality of devices are respectively formed, thus obtaining the individual devices.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines called streets are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby partition a plurality of regions where a plurality of devices such as ICs and LSIs are respectively formed. The semiconductor wafer is cut along the streets to thereby divide the regions where the devices are formed from each other, thus obtaining the individual devices. Further, an optical device wafer is provided by forming a gallium nitride compound semiconductor layer or the like on the front side of a sapphire substrate or a silicon carbide substrate. The optical device wafer is also cut along the streets to obtain individual optical devices divided from each other, such as light emitting diodes and laser diodes, which are widely used in electric equipment.
Cutting of such a wafer including a semiconductor wafer and an optical device wafer as mentioned above is usually performed by using a cutting apparatus called a dicing saw. The cutting apparatus includes a chuck table for holding a workpiece such as a wafer, cutting means for cutting the workpiece held on the chuck table, and feeding means for relatively moving the chuck table and the cutting means. The cutting means includes a rotating spindle, a cutting blade mounted on the rotating spindle, and a driving mechanism for rotationally driving the rotating spindle. The cutting blade is composed of a disk-shaped base and an annular cutting edge mounted on one side surface of the base along the outer circumference thereof. The cutting edge is formed by bonding diamond abrasive grains having a grain size of about 3 μm to the base by electroforming so as to have a thickness of about 15 μm, for example (see Japanese Patent Laid-open No. 2002-66865, for example).
Since the cutting edge of the cutting blade has a thickness of about 15 μm, each of the division lines for partitioning the devices is required to have a width of about 50 μm. Accordingly, in the case that each device has a size of 1 mm×1 mm, for example, the ratio in area of the streets to the devices becomes large to cause a reduction in productivity. Further, when the wafer is cut by the cutting blade, chipping occurs on the cut surface of the wafer on the lower side thereof, causing deterioration in device quality.
As a method of dividing a platelike workpiece such as a semiconductor wafer, there has recently been tried a laser processing method of applying a pulsed laser beam having a transmission wavelength to the wafer in the condition where the focal point of the pulsed laser beam is set inside the wafer in a subject area to be divided. More specifically, this wafer dividing method using laser processing includes the steps of applying a pulsed laser beam having a transmission wavelength to the wafer from one side of the wafer along the streets in the condition where the focal point of the pulsed laser beam is set inside the wafer to thereby continuously form a modified layer inside the wafer along each street and next applying an external force to the wafer along each street where the modified layer is formed to be reduced in strength, thereby dividing the wafer into the individual devices (see Japanese Patent No. 3408805, for example).